Known sensor assemblies typically comprise composite ceramic/metal components that are brazed together using conventional brazing techniques. Such a known sensor assembly might include a metal housing with a metallised aluminium oxide bush brazed into the inner diameter of the housing. A sensor body is then brazed into the internal diameter of the bush.
The sensor body can be made of one or more layers of metal, electrically conductive ceramic, electrically non-conductive ceramic that is made conductive by having a layer of conductive material (e.g. a metal) deposited on its surface, or a conductive ceramic/metal composite, for example. Conductive layers can define electrodes or other sensing elements or shield layers. Non-conductive layers can define insulating spacers that are positioned between conductive layers. The layers that form the sensor body can be machined as a preformed part and then bonded to an adjacent layer or deposited on an adjacent layer using any suitable deposition technique. If the outer layer of the sensor body is made substantially from a ceramic material then its outer surface can be metallised so that the sensor body can be brazed directly into the housing using conventional brazing techniques without the need for the intermediate bush.
The metal housing parts of the sensor assembly might be manufactured from a low expansion alloy which is specifically designed to have a coefficient of thermal expansion substantially similar to that of the bush and/or the sensor body. If the sensor assembly is exposed to high temperatures during operation then the housing, bush and sensor body all expand at similar rates to minimise the thermal stress between the individual components.
One problem with the use of low expansion alloys is that they tend to oxidise at temperatures approaching 500° C. This places an upper limit on the operating temperature of the sensor assembly. It can be difficult to find a metal that is suitable for use at higher temperatures and which also has a thermal expansion coefficient that is substantially similar to that of the bush and/or the sensor body. A known solution is to use so-called “active braze” techniques which allow certain ceramic materials to be brazed to metals without the need for metallised coatings and also provide a degree of compliance between the two different materials to accommodate the different rates of thermal expansion. In practice, however, the operating temperature of active braze alloys is limited to about 800° C. which is still not sufficiently high for certain operations. The compliant coatings that are needed to provide the degree of compliance also tend to oxidise at temperatures below 500° C. and it is normally necessary to provide a hermetic seal at the braze interface to minimise the oxidation effect when the operating temperature falls below this threshold.
Further problems are known to exist in situations where large relative movements occur between the component parts of the sensor assembly as a result of thermal expansion. Large relative movement can only be accommodated by increasing the thickness of the complaint coatings and this can place practical limitations on the design of the sensor assembly.